You refer to this "concern" in the introduction. At this stage, can anything be said about this for version 7? For instance, in the discussions or goal setting, is there anything that referred to this?

As far as I know the changes introduced by PHPNG branch only affect existing extensions that need to be updated, but that is a work that is in progress for extensions maintained by PHP core developers.

As for third-party extensions, those need to be updated by those maintainers.